blk-mq: reduce unnecessary software queue looping

In flush_busy_ctxs() and blk_mq_hctx_has_pending(), regardless of how many
ctxs assigned to one hctx, they will all loop hctx->ctx_map.map_size
times. Here hctx->ctx_map.map_size is a const ALIGN(nr_cpu_ids, 8) / 8.
Especially, flush_busy_ctxs() is in hot code path. And it's unnecessary.
Change ->map_size to contain the actually mapped software queues, so we
only loop for as many iterations as we have to.

And remove cpumask setting and nr_ctx count in blk_mq_init_cpu_queues()
since they are all re-done in blk_mq_map_swqueue().
blk_mq_map_swqueue().

Signed-off-by: Chong Yuan <chong.yuan@memblaze.com>
Reviewed-by: Wenbo Wang <wenbo.wang@memblaze.com>

Updated by me for formatting and commenting.

Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 9 insertions, 4 deletions

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 1192f85e5ff3..0b49e42e5310 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1522,8 +1522,6 @@ static int blk_mq_alloc_bitmap(struct blk_mq_ctxmap *bitmap, int node)
         if (!bitmap->map)
                 return -ENOMEM;
 
-        bitmap->map_size = num_maps;
-
         total = nr_cpu_ids;
         for (i = 0; i < num_maps; i++) {
                 bitmap->map[i].depth = min(total, bitmap->bits_per_word);
@@ -1764,8 +1762,6 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
                         continue;
 
                 hctx = q->mq_ops->map_queue(q, i);
-                cpumask_set_cpu(i, hctx->cpumask);
-                hctx->nr_ctx++;
 
                 /*
                  * Set local node, IFF we have more than one hw queue. If
@@ -1802,6 +1798,8 @@ static void blk_mq_map_swqueue(struct request_queue *q)
         }
 
         queue_for_each_hw_ctx(q, hctx, i) {
+                struct blk_mq_ctxmap *map = &hctx->ctx_map;
+
                 /*
                  * If no software queues are mapped to this hardware queue,
                  * disable it and free the request entries.
@@ -1818,6 +1816,13 @@ static void blk_mq_map_swqueue(struct request_queue *q)
                 }
 
                 /*
+                 * Set the map size to the number of mapped software queues.
+                 * This is more accurate and more efficient than looping
+                 * over all possibly mapped software queues.
+                 */
+                map->map_size = hctx->nr_ctx / map->bits_per_word;
+
+                /*
                  * Initialize batch roundrobin counts
                  */
                 hctx->next_cpu = cpumask_first(hctx->cpumask);